Overall goal: To determine physiological and psychological determinants of the decline in physical activity in Latina and African American girls during puberty. Rationale: The decline in physical activity that occurs during adolescence has been found consistently across gender, ethnicity and nationality in human studies, and across species in animal studies, suggesting a biological basis. The pubertal transition in Latina and African American girls represents a "critical period" of development in which increased insulin resistance and decreased physical activity have been noted. These "risky" metabolic and behavioral changes in this susceptible ethnic group may explain, in part, their increased risk for obesity. This pubertal decrease in physical activity also raises future risk of breast, endometrial and colon cancer. Specific aims and Approach: Aim 1 (Longitudinal Study): To determine the direct impact of pubertal insulin resistance in Latina and African American adolescent girls on physical activity, mood and meanings of physical activity across the pubertal transition from Tanner Stage 1-3. Aim 2: Tc determine how the impact of puberty induced insulin resistance on physical activity is mediated by moot and meanings of physical activity. Aim 3: To investigate ethnic differences in the impact of insulin resistance on mood, meanings of physical activity and physical activity. We will recruit 50 Latina girls and 50 African American girls 9-11 years of age at Tanner Stage 1. Yearly metabolic evaluations and quarterly accelerometry and psychosocial evaluations will be completed for a period of three years. A combination of path models and growth curve models will be used to understand the longitudinal impact of pubertal insulin resistance on mood, motivation and physical activity levels in Latina and African American girls as they mature. Central hypotheses: Pubertal insulin resistance leads to a decline in physical activity. The decline in physical activity in girls is partially biologically programmed, emanating from the "trigger" of insulin resistance, which is linked to affective determinants of physical activity including mood and energy levels, and these metabolic and psychological changes contribute to the marked decline in physical activity that occurs during puberty. This study will be the first to examine the temporal relationship between pubertal insulin resistance and the sharp decline in physical activity experienced by Latina and African American girls during puberty.